Photochromism is a phenomenon which is drawing attention in these several years, and stands for a reversible action of a compound; i.e., a compound quickly changes its color when it is irradiated with light containing ultraviolet rays such as of sunlight or light of a mercury lamp, and resumes its initial color when it is no longer irradiated with light and is placed in a dark place. The compound having such a property is called photochromic compound. Though a variety of compounds have heretofore been synthesized, no common feature is particularly recognized among their structures.
U.S. Pat. No. 4,980,089 discloses a chromene compound represented by the following formula (A), ##STR2##
This chromene compound exhibits photochromic properties near room temperature (20 to 30.degree. C.) but exhibits a slow fading rate when it is placed in a dark place after it has developed a color upon being irradiated with ultraviolet rays. When the colors are mixed upon being combined with photochromic compounds having other color tones, such as a fulgimide and a spirooxazine having relatively fast fading rates, the hue changes when the colors fade.
Furthermore, PCT Unexamined Patent Publication WO95/16215 discloses a chromene compound represented by the following formula (B), ##STR3##
wherein B1 is an aminocarbonyl group or an alkoxycarbonyl group, B2 and B3 are hydrogen atoms, alkyl groups, substituted or unsubstituted phenyl groups or alkoxyl groups, m is from 0 to 3, B4 and B5 are substituted or unsubstituted aryl groups, substituted or unsubstituted heteroaryl groups, substituted or unsubstituted adamantylidene groups, or substituted or unsubstituted norbornylidene groups.
This compound exhibits an increased fading rate compared to the compound disclosed in the above U.S. Pat. No. 4,980,089 but is not still capable of completely preventing a change in the hue when the mixed colors fade. When aged, furthermore, this compound is more colored than when the compound (A) is aged.
PCT Unexamined Patent Publication WO95/16215 further discloses, as a comparative example, a compound represented by the following formula (C), ##STR4##
exhibiting, however, a fading rate nearly the same as that of the above-mentioned compound (A).
As described above, the conventional chromene compounds are not still satisfactory with respect to the fading rate and the color after they are aged.